security: shopping + recipe creation require login

- BottomNav: hide Shopping + Neu when not authenticated
- ShoppingPage: login prompt when not authenticated
- Prevents unauthenticated access to shared shopping data

frontend/src/components/layout/BottomNav.tsx

@@ -1,7 +1,14 @@
 import { NavLink } from 'react-router'
 import { Home, Search, PlusCircle, ShoppingCart, User } from 'lucide-react'
+import { useAuth } from '../../context/AuthContext'
 
-const navItems = [
+const publicItems = [
+  { to: '/', icon: Home, label: 'Home' },
+  { to: '/search', icon: Search, label: 'Suche' },
+  { to: '/profile', icon: User, label: 'Profil' },
+]
+
+const authItems = [
   { to: '/', icon: Home, label: 'Home' },
   { to: '/search', icon: Search, label: 'Suche' },
   { to: '/new', icon: PlusCircle, label: 'Neu' },
@@ -10,6 +17,9 @@ const navItems = [
 ]
 
 export function BottomNav() {
+  const { isAuthenticated } = useAuth()
+  const navItems = isAuthenticated ? authItems : publicItems
+
   return (
     <nav className="fixed bottom-0 left-0 right-0 bg-surface border-t border-sand z-50">
       <div className="max-w-lg mx-auto flex justify-around items-center h-16 sm:h-18">